1. Field of the Invention
The invention relates to temperature control systems and, more particularly, to a thermostat intended to be employed in operating a standard control system of the type generally used in residential or commercial heating and/or cooling systems.
Energy shortages and high costs of fuel have made it increasingly important that heating and air conditioning systems deliver maximum comfort and efficiency for the operating expenses involved. While many suggestions for saving fuel and energy have been advanced, one of the easiest economy measures to implement involves thermostatic control of room temperature. For example, it has been demonstrated that significant fuel savings can be achieved by simply turning down the thermostat at night and many devices for obtaining such control automatically have been proposed.
Another area in which fuel savings can be attained through thermostatic control involves the comfort of individuals and the tendency of individuals to frequently adjust a thermostat during the day when their comfort deviates from that to which they are accustomed. A major cause of variable feelings of comfort is the fact that standard home thermostats of the bimetallic type are frequently responsive only over a wide fluctuation in temperature, as much as 4.degree. F., while a narrow fluctuation, such as 1.degree. F., can be felt by the individual.
2. Description of the Prior Art
Bimetallic element type thermostats having narrow temperature differential sensitivity are available. However, these thermostats tend to be costly in proportion to increased sensitivity while at the same time possessing the disadvantages of mechanical fraility so that they are subject to damage as a consequence of dirt, shock, vibration and the like. Thus, while bimetallic thermostats which offer narrow differential operation to reduce temperature lag and overshoot are available, they tend to be mechanically unstable and unreliable. In addition, the light contact pressure typical in such devices encourages electrical arcing and pitting, adding to the unreliability and promoting relay operated furnace malfunction.
Electronic thermostats are known to overcome some of the disadvantages associated with the bimetallic thermostats. However, prior art thermostats of an electronic nature have employed amplifiers and circuitry which require auxiliary power sources and/or specially adapted relay control mechanisms. As a consequence, component costs and installation expense are frequently too great for acceptability in a residential heating market.
For example, U.S. Pat. No. 3,861,589 (Carlson) discloses an electronic temperature control system and thermostat requiring a differential amplifier. Heat anticipation is obtained, according to the patentee, by providing a diode in contiguous heat transfer relationship with a thermistor. According to the patentee's conditions of operation, thermostat relay current is restricted to 20 mA in the "on state" so that it would not be practical for use with standard home furnace controllers. U.S. Pat. No. 3,623,545 (Pinckaers) discloses a modulating type of electronic thermostat which includes a heat anticipator in the form of a semiconductor current-carrying means that is in heat exchange relationship with the temperature sensor. That is, the patentee uses a component specifically provided and powered to generate heat to be used in anticipation applications. The relatively large amounts of current required to generate an intermediate degree of anticipation in such a device precludes its practical application in a two wire switching system, the type system found in residential applications. U.S. Pat. No. 3,419,214 (Evalds) shows an electronic temperature regulating system employing a DC relay and DC power source, which are not commonly found in the standard residential control system, and a diode compensated bridge which triggers a silicon controlled rectifier (SCR).
Thus, it can be seen that prior art attempts at providing narrow temperature control have been many and varied. These attempts provide varying degrees of success and advantages for particular applications. However, for the reasons described above, a device satisfactory for control of a low voltage relay control mechanism such as would be found on a home oil or gas fired furnace and/or air conditioning or ventilation system has not heretofore been obtained. That is, drawbacks arising from mechanical unreliability and other factors make these devices and systems unattractive to the residential consumer.